Dye diffusion thermal transfer printing is a well known process in which one or more thermally transferable dyes are transferred from selected areas of a dyesheet to a receiver material by localised application of heat, thereby to form an image. Full colour images can be produced in this way using dyes of the three primary colours, yellow, magenta and cyan. Printing is conveniently carried out using a dyesheet in the form of an elongate strip or ribbon of a heat-resistant substrate, typically polyethylene terephthalate polyester film, carrying a plurality of similar sets of different coloured dye coats, each set comprising a panel of each dye colour (e.g. yellow, magenta and cyan plus optional black), with the panels being in the form of discrete stripes extending transverse to the length of the ribbon, and arranged in a repeated sequence along the length of the ribbon.
It is desirable for a dye diffusion thermal transfer printer to be able to determine the characteristics of the dyesheet that has been placed in it. Dyesheets can come in a variety of types and forms which differ in e.g. the type of substrate with which they are suitable for use. Additionally, dyesheets of nominally the same type produced by different manufacturers may have different characteristics.
Many schemes that enable printers to determine the characteristics of particular dyesheets have been proposed. These include the placement of information on a cassette, spindle, or even the dyesheet itself. The information can be stored in a great variety of ways: as an optical, mechanical, resistive or magnetic pattern; in an electronic memory device that can be read electrically or electromagnetically; by fluorescence or resonance; holograms, etc.
The common factor in all of these known schemes is that the information is incorporated and/or read by some additional device or feature that exists solely to carry or read the information. For example, an electronic memory device as described in U.S. Pat. No. 5,455,617 is not only relatively expensive, but it also requires mounting on the media, programming, and specialised components in the printer to access it.
Measurement of the transmitted light intensity is commonly used in printers in order to locate the position of various known panels (for example U.S. Pat. No. 4,496,955) and sometimes to distinguish between types of media (for example U.S. Pat. No. 6,778,200).
EP-A-956 972 discloses a thermal transfer sheet having a plurality of coloured panels and separate identification marks, which may have different transmissivities or reflectivities to each other.